This application pertains to the art of shadowgraphic imaging and more particularly to the art of electronic shadowgraphic imaging. The invention is particularly applicable to shadowgraphic scanners which display a shadowgraphic image on a video monitor and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications such as to scanners which electronically produce printed images, CRT displays, and other representations of shadowgraphic data.
One of the earliest applications of medical X-ray technology was the X-ray shadowgraph. An X-ray source was placed in front of the patient and photographic film placed on the opposite side of the patient. The X-ray source was actuated for a sufficient duration to expose the photographic film. With advances in electronics, improvements soon were made upon X-ray shadowgraphic photography.
For example, as shown by U.S. Pat. No. 2,730,566 to Bartow et al., issued Jan. 10, 1956, it was found that one could produce shadowgraphic images electronically. In the Bartow device, a beam of X-rays is swept through the patient onto an electronic X-ray detector. By sweeping the beam in a pattern which correlates to the electron beam sweep pattern, shadowgraphic images could be produced on a cathode ray tube. Others soon discovered that video displays could be produced from the data collected by sweeping an X-ray beam through the patient. See, for example, U.S. Pat. No. 2,825,817 to North, issued Mar. 4, 1958.
It was also learned that low energy radiation was scattered in the patient's body. Scattered radiation which strikes the film or detector degrades the shadowgraphic image. In film systems, it was found that the amount of scatter could be reduced by placing a beam defining slit between the X-ray source and the patient and another beam defining slit between the patient and the X-ray film. Rather than exposing the entire piece of film at the same time, the two slits were moved in unison such that a narrow band of radiation was swept across the film. Such shadowgraphic scanners with beam defining slits are commonly called slit scanners. It was later found that the use of a plurality of parallel spaced slits allows shadowgraphic images to be produced more quickly. See, for example, U.S. Pat. No. 4,096,391 to Barnes, issued June 20, 1978, and U.S. Pat. No. 4,097,748 to Monvoisin, issued June 27, 1978.
One of the problems with shadowgraphic imaging devices that employ moving slits of radiation is the large energy requirements to which the X-ray tubes are subjected. The amount of radiation produced by an X-ray tube is independent of any beam defining slit with which it may be used. Thus, even if only a relatively small area of the patient is permitted to be irradiated in a scan, the X-ray tube still requires the same amount of energy as it would need without the slit. Moreover, the same exposure time is required to expose a small slit-shaped area as a large area. However, since the number of exposures required to scan a given area is increased if a slit is used, the X-ray tube must be actuated for a proportionately longer duration. By way of example, to form a square image with a slit that is sixteen times as long as it is wide, the slit must be placed at sixteen sequential positions and retained at each such position for an exposure duration. This requires the X-ray tube to be operated for a duration sixteen times as long as it would to expose or image the entire area in one exposure, i.e., without a slit.
Moreover, X-ray tubes are only able to operate at their maximum energy or watt-second rating for a relatively short duration without overheating and hence damaging their anodes. To avoid damaging or melting the anode of the X-ray tube during the extended scanning time required by a slit, the X-ray tube must be operated at a lower power or wattage. However, lowering the power of the X-ray tube, similarly lowers the intensity of radiation produced. To compensate, the slit shaped beam of radiation must dwell in a location for a correspondingly longer duration.
When video medical images are produced today, it is common to divide the examined area of the patient into a matrix of 512 by 512 units or pixels. This number is selected because conventional video monitors have 512 scan lines per frame. Sweeping a slit shaped beam of radiation that has a width of only one 512th of the length of the area to be imaged greatly aggravates the X-ray tube energy problems and extended scanning time problems referenced above. Even if several parallel spaced slits are scanned simultaneously, the X-ray tube would still be called upon to supply many times more power and operate many times longer than practicable.
The present invention contemplates a new and improved shadowgraphic slit scanner which overcomes the above-referenced problems and others by balancing the noted constraints on time of scanning and the power of X-ray tubes in combination with a relatively wide slit to simultaneously expose a plurality of radiation detectors. The inventive scanner further provides electronic or video shadowgraphic images which are substantially free of scatter degradation.